PDLC cell

ABSTRACT

The invention provides a method of filing a PDLC cell, a polymerizable mixture suitable for this purpose as well as a display device provided with such a PDLC cell. The mixture in accordance with the invention comprises two types of non-volatile reactive monomers, the first type of monomer being readily miscible with liquid crystalline material and the second type of monomer being poorly miscible with the liquid crystalline material. Such mixtures prove to be very stable. In addition, when such mixtures are used in cells, problems regarding compositional drift do not occur. Cells in which the inventive mixture is used demonstrate a relatively low hysteresis as well as a relatively low switching voltage. By virtue thereof, it is very attractive to use these cells in a display device.

[0001] The invention relates to a method of manufacturing apolymer-dispersed liquid crystal cell, in which method a mixture, whichpredominantly comprises a liquid crystalline material as well asreactive monomers and a photoinitiator, is sandwiched between twosubstrates, which are provided with an electrode layer, whereafter themixture is polymerized under the influence of radiation. The inventionalso relates to a polymerizable mixture which can suitably be used in apolymer-dispersed liquid crystal cell as well as on a display devicecomprising such a cell.

[0002] Polymer-dispersed liquid crystal cells (abbr. PDLC cells) areincreasingly being used in electro-optic devices, such as displaydevices, optical projectors and electrically drivable optical shutters.The optically active material of these cells is formed by liquidcrystalline material which is dispersed in a matrix of a polymerizedmaterial. Such a material is referred to as a polymer-dispersed liquidcrystalline material (abbr. PDLC material). This material is customarilyprepared by providing a mixture of a liquid crystalline material (70-95% by weight), reactive monomers (5-30 % by weight) and at least onephotoinitiator, in the form of a layer, between two substrates of a celland, subsequently, polymerizing this layer under the influence ofradiation. During polymerization, phase-separation occurs, which leadsto the formation of the desired optically active layer ofpolymer-dispersed liquid crystalline material. This layer can beswitched between an optically transparent state (in the presence of afield) and an optically scattering or translucent state (in the absenceof a field) by means of an electric field.

[0003] A method of the type mentioned in the opening paragraph is knownper se, for example, from European Patent publication EP-A 575.791. Moreparticularly, in examples 5-18 of said publication, a description isgiven of a prepolymer of reactive monomers composed of2-ethylhexylacrylate (EHA) as well as one or two polyfunctionalmonomers. One part by weight of this prepolymer is mixed with four partsby weight of a non-reactive liquid crystalline material. Also a smallquantity of a photoinitiator is added to this mixture. The resultantpolymerizable mixture is subsequently provided between two substratesand polymerized by means of UV light so as to form an optically activelayer.

[0004] The known method has an important drawback. It has been foundthat the electro- optical response of the PDLC cells thus manufacturedis not uniform at all parts of the surface of the cell. For example, theswitching voltage necessary to switch from transparent to scattering,and vice versa, is found to be different for different parts made ofPDLC material. It has further been found that the electro-opticalproperties of the PDLC material are insufficiently stable with respectto time. Life tests show that these properties deteriorate relativelyrapidly. For example, the hysteresis and the switching voltage increaserapidly.

[0005] It is an object of the invention to overcome the above-mentioneddisadvantage. The invention more particularly aims at providing a methodof manufacturing PDLC cells which exhibit a uniform electro-opticalresponse, which is stable with respect to time. The PDLC cellsmanufactured by means of the inventive method should have a relativelylow switching voltage, preferably, of approximately 6 V or less as wellas a relatively low hysteresis, preferably, of approximately 3 % orless. Another object of the invention is to provide a polymerizablemixture which is stable with respect to time and which can suitably beused in the method in accordance with the invention. The inventionshould also provide a display device having an improved PDLC cell.

[0006] These and other objects of the invention are achieved by a methodof the type mentioned in the opening paragraph, which is characterized,in accordance with the invention, in that the mixture comprises twotypes of non-volatile, reactive monomers, the first type of monomerbeing readily miscible with the liquid crystalline material and thesecond type of monomer being poorly miscible with said liquidcrystalline material.

[0007] The invention is based on the insight that in the case of theknown cells a non-uniform electro-optical response is obtained becausethe composition of the PDLC material is not the same everywhere. This isattributed to the presence of EHA in the known polymerizable mixture.This compound has a relatively great volatility. During filing of thecell, this compound evaporates, which leads to concentration differencesin the filled cell. This results in a non-uniform electro-opticalresponse in the known cell. EHA exhibits the greatest volatilityproblems if the cells are filled under the influence of a reducedpressure.

[0008] It has been found that the problem cannot be solved by simplyreplacing the volatile EHA with a single, non-volatile acrylate compoundhaving approximately the same molecular mass. The replacement of EHA ofthe known polymerizable mixture by a non-volatile, higher alkylacrylate,such as decylacrylate (DA) yields poor results. Various electro-opticalproperties, such as the switching voltages and the hysteresis of theswitching curve, of a cell comprising such a polymerized mixture turnout to be considerably worse than those of the known cell comprising theEHA-containing mixture. It is noted that the term “non-volatilemonomers” is to be understood to mean monomers whose vapor pressure issmaller than 1 Pa.

[0009] The invention is further based on the experimentally gainedinsight that the mixing properties of the non-volatile monomers to bepolymerized, which contain the liquid-crystalline material, play animportant role in the electro-optical properties of the ultimate PDLCcell. It has been found that a part of these monomers should be readilymiscible with the liquid-crystalline material, whereas another part ofthese monomers should be poorly miscible with said liquid- crystallinematerial. Mixtures comprising these two types of non-volatile, reactivemonomers can be used very successful in PDLC cells. The electro-opticalproperties of these cells range from good to very good.

[0010] A preferred embodiment of the method in accordance with theinvention is characterized in that the first type of monomer is anethoxylated alkyl-phenolacrylate whose alkyl group comprises at leastfive C-atoms, and in that the second type of monomer is an alkylacrylatewhose alkyl group comprises at least 8 and maximally 18 C-atoms.

[0011] In experiments it has been established that ethoxylatedalkyl-phenolacrylates of the above-mentioned type are very readilymiscible with customary liquid crystalline material, provided that thenumber of C-atoms of the alkyl group is greater than four. It has alsobeen found that alkylacrylates of the above-mentioned type are poorly,i.e. incompletely, miscible with customary liquid crystalline materials,provided that the alkyl group comprises at least 8 and maximally 18C-atoms. If alkyl groups comprising fewer than 8 C atoms are used, thenthe alkylacrylate becomes too volatile. If alkyl groups comprising morethan 18 C-atoms are used, the degree of miscibility of the alkylacrylatewith the liquid crystalline material becomes too high.

[0012] A further preferred embodiment of the method in accordance withthe invention is characterized in that the quantity of each of the twotypes of monomers is at least 20 % by weight, calculated with respect tothe overall quantity of both types of monomers. If the quantity of oneof the two types of reactive monomers is smaller than 20 % by weight,then the switching voltage and the hysteresis of the cell manufacturedwith said monomers is relatively high. Preferably, the ratio betweenboth types of monomers is approximately 1:2. In this case, the lowestvalues as regards switching voltage and hysteresis of the PDLC cell areachieved.

[0013] An interesting embodiment of the method in accordance with theinvention, is characterized in that the mixture is introduced into thecell by means of a reduced pressure. In experiments it has beenestablished that PDLC cells manufactured in accordance with thisembodiment of the invention exhibit very stable electro-opticalproperties.

[0014] The invention also relates to a polymerizable mixture which cansuitably be used in a polymer-dispersed liquid crystal cell, and whichcomprises reactive monomers and a photoinitiator. In accordance with theinvention, this mixture comprises two types of non-volatile, reactivemonomers, the first type of monomer being readily miscible with liquidcrystalline material and the second type of monomer being poorlymiscible with liquid crystalline material. PDLC cells comprising thismixture exhibit good electro-optical properties, such as, in particular,a uniform electro-optical response.

[0015] A preferred embodiment of the polymerizable mixture ischaracterized in that the first type of monomer is an ethoxylatedalkyl-phenolacrylate whose alkyl group comprises at least five C-atoms,and in that the second type of monomer is an alkylacrylate whose alkylgroup comprises at least 8 and maximally 18 C-atoms. In this connection,good results have been achieved with a mixture in which the quantity ofeach of the two types of monomers is at least 20 % by weight, calculatedwith respect to the overall quantity of both types of monomers.

[0016] Preferably, the ratio between both types of monomers isapproximately 1:2. The polymerizable mixture is optimally suitable foruse in a PDLC cell if 70-90 % by weight of a liquid crystalline materialof a customary type has been added. The polymerizable mixture thusobtained can be directly used to fill PDLC cells.

[0017] The invention also relates to a display device comprising apolymer-dispersed liquid crystal cell. In this case, the electrodelayers of the substrates of the cell are constructed so as to form rowsand columns, each row or column being individually drivable. The rows ofone substrate and the columns of the other substrate are oriented so asto extend at right angles to each other. The presence of the matrix ofelectrodes formed by said columns and rows enables pixels of the PDLCmaterial of the display device to be driven locally by means of anelectric voltage.

[0018] Preferably, each one of the pixels is provided with a solid-stateswitch in the form of a thin-flm transistor or a thin-film diode. Byvirtue thereof, it becomes possible to form images. The PDLC cellsmanufactured in accordance with the inventive method can very suitablybe used in such a display device.

[0019] These and other aspects of the invention will be apparent fromand elucidated with reference to the embodiments described hereinafter.

[0020] In the drawings:

[0021]FIG. 1 is a schematic, sectional view of a PDLC cell,

[0022]FIG. 2 shows the electro-optical curve of a PDLC cell,

[0023]FIG. 3 shows a number of structural formulas of chemicalcompounds,

[0024]FIG. 4 shows graphs in which the hysteresis and the V90 or theV50-values of a few mixtures of non-volatile monomers are plotted as afunction of their mixing ratio.

[0025] It is noted that, for clarity, the Figures may not be drawn toscale.

[0026]FIG. 1 is a schematic, sectional view of a cell which can be usedin the manufacture of a PDLC cell in accordance with the invention. Thiscell comprises two predominantly parallel substrates (1, 2) which areprovided with an electrode layer (3, 4) on the surfaces facing eachother. An optically active layer (5) of a polymerized PDLC material issituated in the space between the electrode layers. The distance betweenthe electrode layers ranges between 4 micrometers and 20 micrometers,preferably between 5 and 10 micrometers. This distance is maintained bythe presence of spacers (not shown) in the optically active layer, forexample in the form of small balls or fibers of glass. The space betweenthe electrode layers is closed by means of a seal (6), for example inthe form of a seal line.

[0027] At least one of the electrode layers is made of a transparent,electroconductive material, such as indium-tin oxide (ITO). In order tobe suitable for use in a display device operating in the reflectionmode, at least one of the substrates of the cell is transparent to thelight used. In order to be suitable for use in a display deviceoperating in the transmission mode, both substrates and both electrodelayers of the cell have to be transparent.

[0028] The PDLC cell shown in FIG. 1 was manufactured as follows. Twotransparent substrates, for example of glass, which are provided on amain surface with an electrode layer of ITO are positioned substantiallyparallel to each other by means of spacer balls (7 micrometers across),said electrode layers facing each other. Subsequently, the side faces ofthe space thus formed are sealed by means of a seal line, in such amanner that one or two flitg holes are preserved.

[0029] The cell can be filled in two different ways, i.e. at a reducedpressure or by means of capillary action. If the cell is filled at areduced pressure, said cell is evacuated by means of one or more fiingholes and, subsequently, arranged in the PDLC material to bepolymerized. Subsequently, the vacuum in the cell is removed. Thereduced pressure in the cell, which is created in the above-describedmanner, is used to fil the cell space with the PDLC material to bepolymerized. By contrast, if a cell is filled using capillarity, it isfilled by capillary action instead of a reduction in pressure. If thecell is filled by capillary action, it should comprise at least two figholes, which must be provided in two oppositely arranged parts of thecell.

[0030] After the fig holes have been closed, the PDLC mixture ispolymerized by subjecting it to radiation with UV light (300-400 nm; 7mW/cm²) at a temperature of 30 ° C. for approximately 5 minutes. Duringsaid polymerization process, a phase separation between the liquidcrystalline material and the polymer being formed takes place. Theintended polymer-dispersed liquid crystaline phase is obtained bypolymerizing.

[0031]FIG. 2 shows an example of an electro-optical curve of a PDLCcell. In this curve, the transmission T (%) is plotted as a function ofthe electric voltage V (volt) which is applied across the PDLC layer ofthe cell by means of the ITO electrodes. The arrows (a) and (b) of FIG.2 show the trend of the curve if the voltage increases and decreases,respectively. The VIO and V90-values are also indicated in the curve.The VIO value of the curve is the voltage at which the transmission ofthe ascending curve amounts to 10 % of the transmission which can bemaximally achieved with the cell. The V90 value of the curve is thevoltage at which the transmission of the ascending curve is 90 % of thetransmission which can be maximally achieved with the cell. Thehysteresis of the curve is determined at 50 % of the maximumtransmission. Said hysteresis is indicated as the difference (mV)between the ascending and the descending curve. This value multiplied byhundred and divided by the average value of the ascending and thedescending curve at 50 % transmission is indicated as the percentage (%)of hysteresis.

[0032] Comparative experiment.

[0033] In a first series of experiments, a number of PDLC cells weremanufactured, using a polymerizable mixture in accordance with the stateof the art. The mixture used was composed of 20 % by weight of PN393(Merck) and 80 % by weight of a non-reactive liquid crystalline materialof the type TL205 (Merck). This comprises a mixture of a non-reactive,low-molecular liquid crystalline material. PN393 predominantly comprisesthe reactive monomer EHA, the structural formula of which is shown inFIG. 3. It also comprises a few monomers and oligomers containing two ormore reactive groups. These compounds serve as a cross-linkig agent inthe polymeric material to be prepared. PN393 also comprises twophotoinitiators (Darocur 1173 and Lucirine TPO) to polymerize thereactive groups. In a first series of cells, the mixture was introducedinto the PDLC cells by means of capillary action, and in a second seriesof cells the mixture was introduced via vacuum-fifling, whereafter themixture was polymerized under the above-mentioned conditions.

[0034] Visual inspection revealed that the cells which were filled at areduced pressure exhibited clearly visible ring-shaped structures aroundthe filling hole. Closer inspection also revealed that these cellsdemonstrated a poor electro-optical response. For example, it was foundthat the voltage necessary to switch from transparent to scattering, andconversely, was dependent on the location and hence not the samethroughout the surface of the optically active layer of the cell. Thisphenomenon is ascribed to the so-called “compositional drift”, which canoccur as a result of evaporation of EHA during filling of the cell. Whenthe cell is filled by means of capillary action, said evaporation occursto a lesser degree. The cells of the second series also exhibitedring-shaped structures around the filling hole. However, thesestructures were less clearly visible than those of the vacuum-filledcells.

[0035] Experiments in accordance with the invention.

[0036] In a subsequent experiment, the EHA of the polymerizable mixturePN393 was replaced by a mixture of 37.5 parts by weight of ethoxylatednonyl-phenolacrylate (ENPA, see FIG. 3) and 62.5 parts by weight oftridecylacrylate (DA, see FIG. 3). This mixture is referred to asPN393′. A quantity of 20 parts by weight of this polymerizable mixturewere mixed with 80 parts by weight of the liquid crystalline materialTL205 (Merck). In a first series of cells, the mixture thus obtained wasintroduced into the PDLC cells via capillary-filling, and, in a secondseries, via vacuum- filing, whereafter said mixture was polymerizedunder the above-mentioned conditions.

[0037] Visual inspection of the PDLC cells manufactured in accordancewith the invention revealed that there were no ring-shaped structuresaround the filling opening. In the case of both the vacuum-filed cellsand the capillary-filled cells, said ring-shaped structures were absent.Table 1 lists seven different PDLC cells, which are filled, either viavacuum-filling or capillary action, with one of the two above-describedmixtures, i.e. of PN393 or PN393′. To determine the stability of themixtures, a number of the cells were subjected to an accelerated lifetest after the polymerization process (“aftertreatment”). Table 2 listssome electro-optical properties of these cells, i.e. the value of theswitching voltages V10 and V90 as well as the hysteresis (%). Theseproperties were measured immediately after the manufacture of the cells(cells 1, 3 and 5) or after the cells had been subjected to life tests(cells 2, 4, 6 and 7). TABLE 1 Cell Mixture Filling method Aftertreatment 1. PN393/TL205 Capillary — 2. PN393/TL205 Capillary  5 min,90° C. 3. PN393′/TL205 Capillary — 4. PN393′/TL205 Capillary  60 min,90° C. 5. PN393′/TL205 Vacuum — 6. PN393′/TL205 Vacuum  60 min, 100° C.7. PN393′/TL205 Vacuum 960 min, 100° C.

[0038] TABLE 2 Cell 1 2 3 4 5 6 7 V10 (mV) 5.0 5.7 3.3 3.6 3.1. 3.2. 3.2V90 (mV) 8.8 10.6 6.0 6.4 5.9 6.0 6.1 hysteresis(%) 3.0 7.1 2.6 4.4 2.92.9 2.8

[0039] The Tables show that the cells comprising the polymerizedmixtures in accordance with the invention exhibit a substantially lowerV90 value as well as a substantially higher stability than cellscomprising the known mixture. A comparison between the cells 1 and 2(not in accordance with the invention) shows that after a life test of 5minutes at 90 ° C. the hysteresis of the known mixture has already morethan doubled.

[0040] The hysteresis of cells comprising the mixtures in accordancewith the invention increases less than the hysteresis of cellscomprising the known mixture. The hysteresis of the mixture inaccordance with the invention remains substantially constant if it hasbeen introduced into the PDLC cell via vacuum-filling. Cells which arefilled in this manner also demonstrate the lowest hysteresis and thegreatest stability.

[0041] A comparison of the cells 1 and 2 shows that the V10 and V90values of the known material increase substantially after a shorttreatment at an elevated temperature. The cells filled by means of themethod in accordance with the invention prove to be much more stable inthis respect. The stablest cells are those which are vacuum-filed withthe mixture in accordance with the invention.

[0042] In a number of further experiments, the ratio in which the twonon-volatile reactive monomers occur in the polymerizable mixture wasvaried. In this case, the two non-volatile reactive monomers include theabove-mentioned ENPA with one of the following alkylacrylates:decylacrylate (DA), dodecylacrylate (DDA), tridecylacrylate (TDA) oroctodecylacrylate (ODA).

[0043] The chemical structural formulas of these compounds are shown inFIG. 3. ENPA is a monomer which is very readily miscible with customaryliquid crystalline materials, such as PN393. The above-mentionedalkylacrylates, however, are poorly (i.e. incompletely) miscible withcustomary liquid crystaline materials.

[0044] A quantity of 80 parts by weight of TL205 (Merck) were added to20 parts by weight of the polymerizable mixtures comprising one of theabove-mentioned combinations of non-volatile monomers. The mixtures thusformed were introduced into PDLC cells at a reduced pressure. Afterpolymerization of the mixture under the above-mentioned conditions, thehysteresis as well as the V90 or the V50 value of the cells weredetermined. The measured values are indicated in the graphs of FIG. 4.

[0045]FIG. 4A-C show that the lowest values for the hysteresis and forV90 or V50 are obtained with mixtures comprising at least 20 % by weightof one of the two non-volatile reactive monomers. If use is made ofmixtures comprising less than 20% by volume of one of the twomonofunctional monomers, either the hysteresis or the V90 or V50 valuesdemonstrate an unacceptably large increase. The lowest values areachieved at mixing ratios of the non-volatile monomers of approximately1:2.

[0046] The invention provides a method of filling a PDLC cell, apolymerizable mixture suitable for this purpose as well as a displaydevice provided with such a PDLC cell. The mixture in accordance withthe invention comprises two types of non-volatile reactive monomers, thefirst type of monomer being readily miscible with liquid crystallinematerial and the second type of monomer being poorly miscible with theliquid crystalline material. Such mixtures prove to be very stable. Inaddition, when such mixtures are used in cells, problems regardingcompositional drift do not occur. Cells in which the inventive mixtureis used demonstrate a relatively low hysteresis as well as a relativelylow switching voltage. By virtue thereof, it is very attractive to usethese cells in a display device.

1. A method of manufacturing a polymer-dispersed liquid crystal cell, inwhich method a mixture, which predominantly comprises a liquidcrystalline material as well as reactive monomers and a photoinitiator,is sandwiched between two substrates, which are provided with anelectrode layer, whereafter the mixture is polymerized under theinfluence of radiation, characterized in that the mixture comprises twotypes of non-volatile, reactive monomers, the first type of monomerbeing readily miscible with the liquid crystalline material and thesecond type of monomer being poorly miscible with said liquidcrystalline material.
 2. A method as claimed in claim 1, characterizedin that the first type of monomer is an ethoxylated alkyl-phenolacrylatewhose alkyl group comprises at least five C-atoms, and in that thesecond type of monomer is an alkylacrylate whose alkyl group comprisesat least 8 and maximally 18 C-atoms.
 3. A method as claimed in claim 1,characterized in that the quantity of each of the two types of monomersis at least 20 % by weight, calculated with respect to the overallquantity of both types of monomers.
 4. A method as claimed in claim 1,characterized in that the mixture is introduced into the cell under theinfluence of a reduced pressure.
 5. A polymerizable mixture which cansuitably be used in a polymer-dispersed liquid crystal cell, whichmixture comprises reactive monomers and a photoinitiator, characterizedin that the mixture contains two types of non-volatile reactivemonomers, the first type of monomer being readily miscible with a liquidcrystalline material and the second type of monomer being poorlymiscible with said liquid crystalline material.
 6. A polymerizablemixture as claimed in claim 5, characterized in that the first type ofmonomer is an ethoxylated alkyl-phenolacrylate whose alkyl groupcomprises at least five C-atoms, and in that the second type of monomeris an alkylacrylate whose alkyl group comprises at least 8 and maximally18 C-atoms.
 7. A polymerizable mixture as claimed in claim 5,characterized in that the quantity of each of the two types of monomersis at least 20 % by weight, calculated with respect to the overallquantity of both types of monomers.
 8. A polymerizable mixture asclaimed in claim 5, characterized in that a quantity of 70-90% by weightof a liquid crystalline material is added to the mixture.
 9. A displaydevice comprising a polymer-dispersed liquid crystal cell with a matrixof individually drivable rows and columns of electrodes as well as meansfor driving these electrodes, characterized in that a cell manufacturedin accordance with the method claimed in claim 1 is used in said displaydevice.